ar71xx: add kernel support for TP-Link TL-MR13U

ar71xx: add kernel support for TP-Link TL-MR13U

This patch adds support for TP-Link TL-MR13U router with built-in 10AH
rechargeable battery.

The patch is based on kamwanlai's patch:
https://forum.openwrt.org/viewtopic.php?pid=206746#p206746

Updated to accommodate recent changes introduced with:
ar71xx: remove fs_* variables from image/Makefile

Signed-off-by: Oleg Titov <oleg.titov@gmail.com>
Patchwork: http://patchwork.openwrt.org/patch/4063/
[juhosg:
  - refresh and rename rename 930-add-tl-mr13u-support.patch =>
    625-MIPS-ath79-add-tl-mr13u-support.patch]
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
SVN-Revision: 38063